A moisture curable polymer is a low molecular weight polymer having a functional group cross-linkable with moisture. Before curing, the polymer has such a low molecular weight that it is a liquid at a room temperature and can be poured in a gap, coated on a substrates or shaped in a desired form. Then, it is cross linked and cured with moisture in air to give an elastomeric material. The moisture curable polymer is mainly used as a a sealing material for construction, although it is used as an pressure sensitive adhesive or a molding material.
One of typical moisture curable polmers is a moisture curable polysiloxane having a polysiloxane backbone and at least one reactive silicone group. When it is reacted with water or moisture, a pair of the silicone groups forms a siloxane bond to cure the polymer.
Many moisture curable polymers having a backbone of an organic polymer instead of the polysiloxane have been proposed. Specific examples of such organic polymer are polyoxyalkylene, polyacrylate and the like. Some of such polymers are commercially available. For example, a polymer having a backbone of polyoxypropylene and a methoxysilyl group at its chain end is distributed in a trade name of "MS Polymer" from Kanegafuchi Chemical Industry, Co., Ltd.
Not only the organic polymer having the reactive silicone group is cheaper than the polysiloxane but also the former has many useful properties which are not found in the latter depending on the kind of the backbone organic polymer. However, the organic polymer having the reactive silicone group still has some drawbacks to be overcome.
Since one of the most important properties of the elastomer is elongation, the organic polymer having the reactive silicone group is required to have large elongation. Improvement of elongation of the elastomer is achieved by increasing a chain length between the cross linking sites. Although it is possible to increase the chain length between the reactve silicone groups, some other problems are caused. That is, increase of the molecular weight render the polymer viscous or solid. One example will be shown in below.
Polymethacrylate or polyacrylate having the reactive silicone groups is produced by one of following methods:
1. A method co-polymerizing a monomer having a reactive silicone group such as vinyltrimethoxysilane and .gamma.-methacryloxypropyltrimethoxysilane.
2. A method utilizing a chain transfer agent such as mercaptosilanes (e.g., .gamma.-mercaptopropylmethyldimethoxysilane and .gamma.-mercaptopropyltriethoxysilane) and disulfides having a reactive silicone group e.g, bis-.gamma.-methyldimethoxysilylpropyldisulfide and bis-.gamma.-triethoxysilylpropyldisulfide).
3. A method utilizing a pclymerization initiator having a reactive silicone group (e.g., .alpha.,.alpha.'-azobis-5-trimethoxysilyl-2-methyl-valeronitrile).
A polymer produced by one of the above method or by combination of two or more of them has unsatisfactory elongation since, if the produced polymer has a low molecular weight and is liquid at a room temperature, a cross linked polymer has a short chain length between the cross linking sites. Thus, to obtain a cross linked polymer having satisfactory elongation, it is necessary to increase the molecular weight of the polymer so as to increase the chain length between the cross linking sites. Since the high molecular polymer is highly viscous or solid, it should be dissolved in a large amount of a solvent to decrease viscosity of a solution when it is used as a liquid sealing material or an adhesive. However, a solution type sealing material will shrink on evaporation of the solvent or has inferior adhesivity due to the solvent Further, when a solvent type adhesive is coated on a substrate, it takes longer time to evaporate the solvent. In addition, the solvent would cause fire or is harmful to human beings.
According to the conventional methods, it is difficult to produce a moisture curable organic polymer which is a less viscous liquid before curing and cured to give an elastomer with large elongation. In other words, it is difficult to produce a moisture curable polymer which has a low molecular weight before curing and cured to give an elastomer having a long chain length between the cross linking sites.